Two heliostats installed on the Golden Gate Bridge towers have been flashing bright sunlight around San Francisco Bay since the bridge’s May 27 birthday celebration, and now members of the public can go online to schedule their own private viewing.

The mirror-assemblies, dubbed Solar Beacon, were built by a team of University of California, Berkeley, space scientists and mounted by bridge engineers on May 25. They flashed sunlight at Crissy Field during the birthday ceremonies, and have been flashing at UC Berkeley’s Lawrence Hall of Science every day since, with frequent spotlights on tourist spots in San Francisco and the Berkeley marina.

Even on a hazy day, Solar Beacon shines brightly as seen from UC Berkeley’s campanile (Sather Tower). (Video produced by Roxanne Makasdjian)

Through the website http://solarbeacon.org/, anyone can now schedule a time for the heliostats to swivel at just the right angle to flash four minutes of sunlight at a specific spot, as long as that spot is within sight of the tops of the towers. Residents of the East Bay can catch the sun in the morning and early afternoon. Those in Marin County can see the solar beacon best during the morning and evening, while people in San Francisco can schedule a viewing at pretty much any time of the day.

The heliostats – each about two feet across and consisting of four six-inch-square mirrors – are an art installation proposed by UC Berkeley space scientist John Vallerga to celebrate the 75th anniversary of the completion of the bridge. It was one of 75 birthday projects approved by the Golden Gate Bridge District, and one of few actually mounted on the bridge.

Each mirror assembly is attached to motors that can swivel and tilt to send a reflected image of the sun anywhere within view of the tops of the orange towers. The image of the sun is about one-half degree across, which means it will be spread out to more than 90 feet at a distance of about two miles. People will see a bright point of light on the top of each tower, similar in brightness to the sun reflecting off a car mirror.

At the Solar Beacon website, the public cannot only schedule a steady, four-minute reflection, but program the mirrors to flicker on and off in a predetermined pattern.